A solution that worked for me (with the same problem while watching a YouTube video) was to right mouse-click the top left hand corner of the tab. If the problem appears while you're watching a YouTube video (i.e. You get video but no sound) when you hover your mouse over the top left hand corner of your tab you'll see that the YouTube icon changes into muted speaker icon. Opera has a built-in feature that allows you to mute the audio playing in a tab by right mouse-clicking the top left hand corner of the tab. To unmute the audio just right mouse-click again. Problem solved. Good luck!